Natalizumab (NMAB) is a humanized antibody that binds to surface-expressed integrins on all leukocytes except neutrophils, and inhibits adhesion molecules on these leukocytes from binding to their counter-receptors. In so doing, NMAB disrupts the transmigration of these leukocytes across endothelial tissue and into inflamed parenchymal tissue. NMAB additionally inhibits recruitment and inflammatory activity of activated immune cells. Although the exact mechanism of action of NMAB is unclear, it is thought that at least part of the therapeutic benefit of NMAB in the treatment of MS is due to NMAB's ability to decrease the number of activated inflammatory cells, including T-lymphocytes, across the blood-brain barrier (BBB), thus decreasing the degree of inflammation within the CNS.
MS is considered to be a chronic inflammatory disease of the CNS, which is often due to myelin sheath and axonal CNS damage secondary to an autoimmune inflammatory process that usually includes a T-cell response. A more virus or toxin related demyelination has also been identified as a primary disorder within oligodendrocytes. In double blind, multi-center, placebo controlled trials, a therapeutic benefit of NMAB has been observed in patients with MS; and this is, at least in part, attributed to decreasing the extent of T-lymphocyte entry across the BBB and into the CNS, and therefore decreasing pathologic inflammatory disease within the CNS.
Unfortunately, in 2005 it was observed that three patients who had been treated with NMAB were diagnosed with Progressive Multi-focal Leukoencephalopathy (PML), a rare and often fatal disease of the brain that has been observed historically in immunosuppressed patients. While it is not definitively clear at the time of this application whether or not the diagnosed PML was caused by treatment with NMAB or simply associated with same, the inventors' understanding of the etiology of PML leads them to believe that the former is more likely. PML is progressive (over time it continues to encroach more and more brain tissue), multi-focal (occurs in more than one location within the brain), and is a disease of the white matter of the brain (leukoencephalopathy). Like MS, PML causes demyelination and can result in severe and often fatal neurological injury. The etiology of PML is understood to originate from a virus, the JC Virus, which can infect and kill oligodendrocytes, which are specialized neural cells that produce the myelin essential for proper neuronal function.
While still rare, the resurgence of PML during the past two decades was in part due to the compromised T-cell mediated immunity associated with patients with AIDS. In short, treatment with NMAB results in a CNS immunosuppression of sorts, akin (at least form the point of view of the CNS) to the more ubiquitous T-cell immunosuppression observed in patients with AIDS. It has also been observed that there is an association between JC Virus-specific cytotoxic T lymphocytes (CTL) and the early control of PML. That is, the more JC Virus-specific CTL, the more favorable outcome in patients with PML.
Although far from universal, prolonged PML survival after Highly Active Antiretroviral Therapy (HAART) (against the AIDS virus) has been reported and prolonged survival has been observed with JC Viral clearance from the CSF.